1. Field of the Invention
The invention relates to a method for monitoring a transmitter. The transmitter comprises at least one measurement unit and one transmission unit. The measurement unit and the transmission unit are interconnected at least for transmission of signals. Furthermore, there is at least one connecting line for supplying power to the measurement unit which is used to determine at least one measurement quantity and to produce a measurement signal which is dependent on it. The transmission unit receives the measurement signal from the measurement unit and is used to transfer an output signal to at least one signal transmission element, based on the measurement signal. The invention also relates to a corresponding transmitter.
2. Description of Related Art
Transmitters—alternative names are transducers or field devices—of the aforementioned type have been known for a long time and are used quite generally to detect the primary sensor signal which originates from a sensor unit or a sensor element and to convert it into a generally standardized signal as an electrical output signal, so that it can be used for example, by a higher-level process monitoring unit or control unit. The term transmitter should not be understood as limiting in any way. The sensor unit can be used especially for measurement of physical or chemical measurement quantities, for example, flow rate, mass flow rate, pressure, temperature, fill level, pH value, etc.
This transmitter can be divided at least into two parts for the general examination: into a measurement unit which is used for the measurement in itself and which makes available a measurement signal which originates from the measurement, and into a transmission unit which receives the measurement signal and transfers it for example, via a field bus to a higher-level unit. One part of the transmitter performs the task of actual measurement and the other part of the transmitter communicates the measurement value which has been obtained in the measurement or optionally the acquired measurement values to other units.
Transmission takes place via any signal transmission element which is, for example, an interface of a field bus or a two-wire connection site or any type of a field bus itself or any communications element or line element for a 4 . . . 20 mA signal, for example. In one example, it is an electrical conductor. It can also be, for example, a wireless radio link.
Depending on the application, transmitters must satisfy different safety requirements. Thus, in particular, possible error sources must be recognized, and optionally, there must be safety measures. For errors, critical regions are, for example, the measurement itself, the signal processing or data processing in the transmitter and also the communication with for example, the units which are at a higher level than the transmitter. In order to satisfy the respective safety requirements, for example, the SIL (Safety Integrity Level) standard which is important especially in process automation, there must be, for example, redundancy or diversity for the transmitters. Redundancy here means a doubled or multiple layouts of safety-relevant components. Diversity means that the hardware components or software programs which are used originate from different manufacturers or are of different type. Both redundant and also diverse configurations are generally complex and/or costly.
For example, the European Patent EP 1 466 308 B1 and corresponding U.S. Pat. Nos. 7,139,683 B2 and 7,539,600 B2 disclose a sensor arrangement with a measurement detector which generates a raw signal, and with an output stage which outputs an output signal. Between the measurement detector and the output stage, transmission and conversion of the raw signal into the output signal take place. An additional monitoring unit generates, from the raw signal, an auxiliary signal and compares it to the output signal in order to signal deviations beyond a predetermined framework. It is disadvantageous that there must be a second unit for processing of the raw signal. At the same time, only the signal path within the transmitter is monitored in this way.
U.S. Pat. No. 6,017,143 describes a transmitter with a measurement unit and an input/output unit for connection to a process loop. Furthermore, there is a microprocessor which is connected to a storage unit. Data and rules which have been taught or input are stored in the storage unit. The microprocessor is designed such that it recognizes and signals errors based on the stored data and the measurement signals.
European Patent Application EP 2 219 013 A1 and corresponding U.S. Patent Application Publication 2010/0211342 A1 disclose a circuit via which it can be checked in a measurement device whether a set measurement current corresponds to a setpoint.
U.S. Patent Application Publication 2005/0189017 A1 discloses a pressure sensor which checks the measurement signals for errors and signals the presence of an error.
A quantity which is relevant for assessing a transmitter with respect to safety is the safe failure fraction (SFF) which indicates how large a portion of the safe error of the altogether possible errors exists. A safe error is an error which is relevant to safety, and which however, either is recognized or transfers the transmitter into a safe state. Therefore, one problem lies in the detection of the presence of an error or an error state. Another problem is the safe communication of the detected error or of the shifting of the transmitter into a safe state as easily as possible.